Edited by Jill M. Slade, Michigan State University, East Lansing, MI, and accepted by Editorial Board Member C. O. Lovejoy March 31, 2017 (received for review November 29, 2016)
Chimpanzee “super strength” has been widely reported since the 1920s although a critical review of the available data suggests that the chimpanzee–human muscular performance differential is only ∼1.5 times. Some hypothesize that this differential reflects underlying differences in muscle mechanics. Here, we present direct measurements of chimpanzee skeletal muscle properties in comparison with those of humans and other terrestrial mammals. Our results show that chimpanzee muscle exceeds human muscle in maximum dynamic force and power output by ∼1.35 times. This is primarily due to the chimpanzee’s higher fast-twitch fiber content, rather than exceptional maximum isometric force or maximum shortening velocities. We suggest that muscular performance capabilities declined during hominin evolution in response to selection for repetitive, low-cost contractile behavior.
Since at least the 1920s, it has been reported that common chimpanzees (Pan troglodytes) differ from humans in being capable of exceptional feats of “super strength,” both in the wild and in captive environments. A mix of anecdotal and more controlled studies provides some support for this view; however, a critical review of available data suggests that chimpanzee mass-specific muscular performance is a more modest 1.5 times greater than humans on average. Hypotheses for the muscular basis of this performance differential have included greater isometric force-generating capabilities, faster maximum shortening velocities, and/or a difference in myosin heavy chain (MHC) isoform content in chimpanzee relative to human skeletal muscle. Here, we show that chimpanzee muscle is similar to human muscle in its single-fiber contractile properties, but exhibits a much higher fraction of MHC II isoforms. Unlike humans, chimpanzee muscle is composed of ∼67% fast-twitch fibers (MHC IIa+IId). Computer simulations of species-specific whole-muscle models indicate that maximum dynamic force and power output is 1.35 times higher in a chimpanzee muscle than a human muscle of similar size. Thus, the superior mass-specific muscular performance of chimpanzees does not stem from differences in isometric force-generating capabilities or maximum shortening velocities—as has long been suggested—but rather is due in part to differences in MHC isoform content and fiber length. We propose that the hominin lineage experienced a decline in maximum dynamic force and power output during the past 7–8 million years in response to selection for repetitive, low-cost contractile behavior.
Author contributions: M.C.O., B.R.U., and P.J.R. designed research; M.C.O., N.B.H., S.G.L., and P.J.R. performed research; M.C.O., B.R.U., and P.J.R. contributed new reagents/analytic tools; M.C.O. analyzed data; and M.C.O., B.R.U., N.B.H., S.G.L., and P.J.R. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. J.M.S. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1619071114/-/DCSupplemental.
